Space Station Experiments Could Prove that Dark Matter Destroys Itself

The AMS-02 instrument, shown here attached to the outer hull of the ISS. Credit: NASA

A scientist has attempted to prove the evidence of dark matter since it’s existence was first suggested 50 years ago to explain the so-called ‘missing mass’ of the Universe. The invisible matter supposedly accounts for almost 30% of visible matter. Still, in spite of continual research, the scientist can find site nor hair of the mysterious mass.

Yet, a pair of new papers, published in Physical Review Letters, has announced that experiments performed on the International Space Station might have given a glimpse of the elusive matter. The AMS-02, also known as the Alpha Magnetic Spectrometer, is a top of the line particle detector taking recordings of cosmic rays, possibly caused by the destruction of dark matter, for 6 years.

Similar to the AMS, the previous model, the AMS-02 was produced by a globe-spanning team from 56 institutions in 16 different countries and was sponsored by the US Department of Energy under the guidance of the Johnson Space Center’s AMS Project Office. The AMS-02 was brought aboard the ISS by the Space Shuttle Endeavor where it has resided since May of 2011.

Artist’s impression of the AMS-02 instrument. Credit: NASA/JSC

Originally designed to track cosmic rays, the AMS-02 measures a number of antiprotons descending to Earth. However, the research teams behind the papers were also analyzing the collected data and testing theories concerning dark matter, specifically the WIMPS theory.

The WIMPS theory posits that protons and antiprotons are created as the result of the Weakly-Interacted Massive Particles, or WIMPs, that compose dark matter, colliding. By observing the number of antiprotons picked up by the AMS-02, both teams, working independently, wanted to know if they could tell whether the antiprotons were created by WIMP strikes. However, the task was not an easy one as cosmic rays originate from a number of sources and the defining properties of WIMPS are not fully known.

In order to interpret this data, both teams formulated mathematical systems that would predict the background cosmic ray, and, therefore, constrain a number of antiprotons that AMS-02 could read. They additionally included precise estimates of expected WIMPs mass that matched the previously collected AMS-02 data.

A team, led by Alessandro Cuoco from the Institute for Theoretical Particle Physics and Cosmology, used computer simulations to study the AMS-02 data under two distinct sets of circumstances- one accounting for dark matter, the other did not. At the end of the study, they found both that the existence of antiprotons made with WIMP crashes matched the data and that the dark matter mass was around 80 GeV, 85 times the mass of a proton or an antiproton.

The paper states: “[T]he very accurate recent measurement of the CR antiproton flux by the AMS-02 experiment allows [us] to achieve unprecedented sensitivity to possible DM signals, a factor ~4 stronger than the limits from gamma-ray observations of dwarf galaxies. Further, we find an intriguing indication for a DM signal in the antiproton flux, compatible with the DM interpretation of the Galactic center gamma-ray excess.”

The second research team was composed of members from various Chinese universities and institutes and led by Ming Yang Cui of Nanjing University. This team created their background cosmic ray estimates based on boron to carbon ratios and proton readings from previous data sets.

By measuring the rate that boron decays into carbon as a gauge for how far the molecules have moved in space and combining it with the previous proton measurements, the research team defined cosmic ray background levels. They added their data to a Bayesian Analysis framework, a model used to ascertain probabilities, hoping to identify the number of antiprotons that could be ascribed to WIMP strikes.

The Chinese team’s results mirrored those of Cuoco’s team. “Compared with the astrophysical background only hypothesis, we find that a dark matter signal is favored,” they stated in their paper. “The rest mass of the dark matter particles is ?20 – 80 GeV.”

According to supersymmetry,dark matter particles known as WIMPs annihilate each other, creating a cascade of particles and radiation. Credit: Sky & Telescope / Gregg Dinderman.

There were more similarities in the results from the independent teams. Cui’s team estimated a dark matter cross-section measurement, something that predicts the probability of the collisions by examining the density of dark matter distribution, in a range from 0.2 — 5 x 10-26 per cm³, while Cuoco’s team separately estimated a cross section of 3 x 10-26 per cm³.

Although not conclusive evidence of the existence of dark matter, it is encouraging that separate research groups, working independently, created similar results from the same information. If nothing else, they are nearer to forming a complete concept of dark matter than before. More work is need as Cuoco suggests: “Confirmation of the signal will require a more accurate study of the systematic uncertainties, i.e., the antiproton production cross-section, and the modeling of the effect of solar modulation.” All of this new evidence is only possible due to the acute readings of the ASM-02, which surpasses the sensitivity of any other instrument before it.